Area Forecast Discussion
Issued by NWS Sterling, VA

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497 FXUS61 KLWX 100309 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1009 PM EST Sat Dec 9 2017 .SYNOPSIS... Low pressure along the Delmarva coast will continue moving northeastward and away from the region overnight. A cold front will cross the region overnight. Several weak disturbances will cross the region early next week and then a shot of arctic air arrives by mid week. && .NEAR TERM /THROUGH SUNDAY/... As the coastal low moves northeast and away from the region, the last of the snow is over the eastern shore and east of our area. See latest Public Information Statement as of 927PM containing snow totals received in our office since 630pm. Overall this snow event was complicated by the very strong jet over New England which meant minimal development of the coastal low. But the position of exit/entrance regions of the upper level east coast jet allowed more elongated and narrow snow bands to occur further inland...with north central Maryland getting the most snow. Predicting the exact location of these bands was challenging and the jet position allowed for some pretty decent banding to occur. Fortunately, ground temperatures going into this event were above freezing and pretty much stayed that way throughout the event which precluded much accumulations on roads and lessened the storm`s impact. Another interesting aspect of this snowfall was the unexpectedly high snow-liquid ratio (SLR) values observed. Here at the office our 4.0 and melted equiv of 0.21 inches gave SLRs here of near 20 to 1, which looking at the IAD 12Z/09 and 00Z/10 soundings is reasonable...with lots of cold air aloft and decent instability in the favored dendritic growth layer. Surface temperatures also were a bit colder throughout the day compared to earlier model forecasts. Overnight, clouds will eventually thin out and temperatures will remain below freezing. This could cause hazardous icy conditions on any untreated roads, parking lots or sidewalks during the overnight and early Sunday morning. Another issue overnight is upslope snow in the western mountains where 1-3 inches snow accumulations in snow showers are expected during the overnight. This snow activity is associated with a upper level vort that will help sweep the cold front across the region overnight. High pressure will build across the region on Sunday with with dry weather expected. Temperatures will climb above freezing by late morning most locations and breezy westerly winds along with fresh snow cover will make if feel and look quite winter-like. && .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... A weak shortwave will pass to our north Sunday night, potentially bringing some upslope snow showers to the favored locations, but accumulations looking unlikely. Temps on Monday will moderate a bit, however remaining below normal in the lower 40s. Conditions will remain dry under partly cloudy skies. Monday night another clipper system will cross the Great Lakes region, dragging a cold front toward the Mid-Atlantic. Threat of wintry precipitation seems to be low at this time compared to earlier guidance, with the potential for some snow showers across our western slopes and light/mixed wintry precipitation possible across the far northern sections of our CWA. && .LONG TERM /TUESDAY THROUGH SATURDAY/... A highly amplified upper level flow pattern will continue to dominate most of the long term period with west coast ridging and east coast troughing. Low pressure will be passing north of the area early Tuesday as a shortwave trough digs southeastward toward the area. If timing remains consistent, highs in the 40s could occur early in the day, followed by cold advection behind the departing cold front. While snow will be most likely along and west of the Allegheny Front, we couldn`t totally rule out some snow showers or flurries making it farther east as well. It will also become breezy, which will persist through Wednesday in tight northwest gradient. Thermodynamic profiles become more favorable for flurries/snow showers on Wednesday as additional vorticity energy rotates through the trough. Temperatures have the potential to be quite cold Tuesday night through Thursday night -- on the order of 15 degrees below normal -- as the trough is at its deepest. Weak high pressure may nudge into the area early Thursday, but attention will then turn to the next shortwave trough diving out of central Canada sometime late Thursday into Friday. Model spread increases during this time, but some light snow is not out of the question depending on where the best forcing and moisture track. The pattern may finally buckle by Saturday, allowing high pressure to build across the area and temperatures to return closer to seasonal averages. && .AVIATION /03Z SUNDAY THROUGH THURSDAY/... Some MVFR conditions persist this evening with visibility and CHO has a low LIFR deck. Light winds for the next several hours will persist before winds kick up from the NW later during the overnight with the cold front. Gusty NW winds expected from Sunday morning with gusts upwards of 25 knots. VFR conditions expected Monday, with sub-VFR possible late Monday night into early Tuesday morning with light wintry precipitation. Winds will be light out of the south Monday into Monday night. Mainly VFR expected Tue into Thur with only brief IFR impacts in passing snow showers possible Tuesday through Wednesday. W/NW wind gusts greater than 30 kt will also be possible during this time. Weak high pressure will likely bring calmer conditions early Thursday. && .MARINE... Northwest winds kick in after midnight with SCA conditions expected that continue through Sunday, and could linger into Sunday night. Light winds are expected Monday before winds increase again ahead of a clipper late Monday night. A cold front will push through early Tuesday, bringing increasing west/northwest winds which will persist through Wednesday. High end Small Craft Advisory conditions appear likely, and there could be a period of gales. Weak high pressure should bring diminishing winds late Wednesday night into Thursday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...Winter Weather Advisory until 9 AM EST Sunday for VAZ503. WV...Winter Weather Advisory until 9 AM EST Sunday for WVZ501-505. MARINE...Small Craft Advisory until 6 PM EST Sunday for ANZ530>533-535- 536-538>542. Small Craft Advisory until 6 PM EST Sunday for ANZ534-537-543. && $$ SYNOPSIS...SMZ NEAR TERM...SMZ/RCM SHORT TERM...BKF LONG TERM...ADS AVIATION...SMZ MARINE...SMZ/BKF

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